1. Field of the Invention
The present invention relates to a tertiary mode vibration type Coriolis flowmeter provided with at least one flow tube.
2. Description of the Related Art
In a Coriolis flowmeter, a tube through which a fluid to be measured flows is supported at one end or both ends thereof, and vibration is applied to a portion of the tube around the supporting point in a direction vertical to the flowing direction of the tube (hereinafter, a tube to which vibration is applied is referred to as a flow tube). The Coriolis flowmeter is a mass flowmeter, which utilizes the fact that the Coriolis forces applied to the flow tube when vibration is thus applied thereto, are proportional to a mass flow rate. The Coriolis flowmeter, which is well known, is roughly classified into a straight tube type and a bent tube type in terms of flow tube structure.
In a Coriolis flowmeter of the straight tube type, when vibration is applied to a straight tube, whose both ends are supported, in a direction vertical to the straight-tube center portion axis, a difference in displacement due to the Coriolis forces is generated between the support portions and the central portion of the straight tube, that is, a phase difference signal is obtained, and, based on this phase difference signal, the mass flow rate is detected. The straight tube type Coriolis flowmeter, thus constructed has a simple, compact, and solid structure. On the other hand, there arises a problem in that it is difficult to achieve high detection sensitivity.
In contrast, the bent tube type Coriolis flowmeter is superior to the straight tube type Coriolis flowmeter from a view point that it allows selection of a shape for effectively obtaining the Coriolis forces. In fact, it is capable of performing mass flow rate detection with high sensitivity. Known examples of the bent tube type Coriolis flowmeter include one equipped with a single flow tube (see, for example, JP 4-55250 A), one equipped with two flow tubes arranged in parallel (see, for example, JP 2939242 B), and one equipped with a single flow tube in a looped state (see, for example, JP 2951651 B).
A Coriolis flowmeter provided with a single flow tube is advantageous in that it can provide a mass flowmeter in a simplest structure and at low cost. On the other hand, in the Coriolis flowmeter provided with a single flow tube, it has a problem in that, when the flow tube is vibrated in a primary mode or an even-number mode, the vibration leaks to an exterior of the mass flowmeter.
To mitigate the vibration leakage to the exterior of the mass flowmeter, there has been devised, as means for mitigating leakage of the vibration of the flow tube to the exterior, a construction in which the single flow tube is vibrated in a tertiary mode by using three drive devices. However, when the three drive devices are used, a number of new problems occur. For example, the wiring for the three drive devices is rather complicated, and the production cost is increased.